Investment casting is one of the oldest known metal-forming processes, dating back thousands of years to when it was first used to produce detailed artwork from metals such as copper, bronze and gold. Industrial investment castings became more common in the 1940's when World War II increased the demand for precisely dimensioned parts formed of specialized metal alloys. Today, investment casting is commonly used in the aerospace and power industries to produce gas turbine components such as airfoils having complex outer surface shapes and internal cooling passage geometries.
The production of a component using the prior art lost wax investment casting process involves producing a ceramic casting vessel including an outer ceramic shell having an inside surface corresponding to the desired outer surface shape of the component, and one or more ceramic cores positioned within the outer ceramic shell corresponding to hollow interior passages to be formed within the component. Molten metal alloy is introduced into the ceramic casting vessel and is then allowed to cool and to solidify. The outer ceramic shell and ceramic core(s) are then removed by mechanical or chemical means to reveal the cast component having the desired external shape and hollow interior volume(s) in the shape of the ceramic core(s).
Certain component designs may include a dual wall structure wherein two regions of metal are separated by a hollow space, as may commonly be used for internally cooled hot gas path components of a gas turbine engine. FIG. 1 illustrates this concept for a simple rod-in-a-tube component 10, although one skilled in the art will recognize the application of this concept to more complex structures where the inner structure has a more planar geometry, such as is often used for internally cooled gas turbine hot gas path components. In cross-section, component 10 includes an outer tube wall 12 encircling an inner rod (wall) 14, thereby defining an open volume 16 there between. The metal alloy component 10 may be cast using a hollow ceramic core 20, as illustrated in FIG. 2. The ceramic core 20 defines the shape of the open volume 16 when the component 10 is cast within an outer casting shell (not shown).
It is known to form the hollow ceramic core 20 of FIG. 2 by first producing a consumable preform 22, as shown in cross-section in FIG. 3, which is formed of wax. The wax preform 22 is then placed into a mold 24 and ceramic slurry 26 is injected around the preform 22. The ceramic slurry 26 is dried to a green state and then removed from the mold 24 and placed into a furnace for firing of the green body to form the ceramic core 20. The green body may be externally mechanically supported within the furnace by a packing material during the firing process. Such packing material may be a ceramic powder which is not subject to sintering during the firing sequence. The wax preform 22 will melt early in the firing sequence and will puddle, and it will eventually volatize and be removed from the furnace as a gas. It is known that such hollow ceramic molds 20 are often difficult to produce and subject to distortion, breakage and low yields because the green body strength of the dried but unfired ceramic slurry 26 is low, and it remains unsupported on its interior surface 28 once the wax preform 22 melts. Prior to the wax melting, there may be deleterious differential thermal expansion forces imposed on the green body due to the different coefficients of thermal expansion of the wax and ceramic materials.
Accordingly, an improved process for forming dual-walled and hollow cast metal components is desired.